Examples of using supplementary magnetic fields to correct MRI magnetic field homogeneity are disclosed in U.S. Pat. No. 6,968,982, and Wen Z., et. al, Shimming with Permanent Magnets for the X-Ray Detector in a Hybrid X-Ray/MR System, 35(9) Med. Phys. 3895 (2008), available at http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2673662/.
During magnetic resonance imaging (MRI), magnetic fields can be induced in non-biological materials within a patient such as medical implants or orthodontic appliances (dental braces). Non-biological materials can be magnetized by the strong magnetic field of an MRI scanner, and the induced magnetization in the non-biological materials can become the source of a non-uniform magnetic field. These induced magnetic fields can disrupt MRI magnetic field (B0) homogeneity and cause image intensity losses in regions near the non-biological materials and geometric distortions across the image. For example, during an MRI of the brain, non-biological materials located within the mouth of a patient may cause image intensity loss in the oral cavity and geometric distortions over the whole brain, specifically the orbits, hypothalamus, Circle of Willis, frontal lobe, and temporal lobes. Image loss and/or distortion is most severe for diffusion-weighted images, gradient echo images, and magnetic resonance angiography, and there is a loss of spectral resolution in magnetic resonance spectroscopy, as these techniques require a high degree of B0 homogeneity.
Approximately 40% of the general population wears orthodontic appliances at some point in their life, particularly during adolescence. Many, if not most, dental braces comprise a common non-biological material which may be found within a patient's mouth. More particularly, approximately 95% of dental braces include brackets that comprise ferromagnetic stainless steel, as stainless steel is low cost, high strength, and durable. While orthodontic appliances, as well as other surgical implants, may be made of other more MRI-friendly materials such as plastic or titanium, these materials tend to be either too weak or expensive [19-32, 43-44]. Dental braces are particularly common in children, causing concern for children's hospitals, since around 80% of MRIs performed in a children's hospital involve imaging of the brain. For patients with dental braces, diffusion-weighted images and magnetic resonance angiography images are generally incomprehensible and therefore not performed. Such imaging techniques are critical for diagnosing many serious brain conditions, such as stroke.
One method for remedying the effect of non-biological materials on MRI imaging is to remove the non-biological materials from the patient before performing an MRI. In the case of dental braces, this involves removing the patient's braces prior to performing the MRI. Often times, as is the case with dental braces, removal of the non-biological materials is time consuming, expensive, and may be unavailable in emergency or after-hours situations. Other times, the MRI scan is performed with the non-biological materials in place, resulting in suboptimal image quality. In tumor patients requiring frequent follow-up MRI scans, removal of braces may lead to premature termination of orthodontic treatment. Current MRI technology seeks to remedy B0 inhomogeneity through a technique known as image shimming, and most if not all scanners (e.g., 1.5 T (1.5 tesla) scanners) are capable of shim (e.g., linear shim). However, shimming is unable to remove image artifacts caused by non-biological materials within a patient. Other approaches include software correction [33-35], pulse sequence design and optimization [36-42], image unwrapping techniques [50], and sequence segmentation [51], which may be helpful for certain types of scans. However, these techniques generally fail to directly address inhomogeneities caused by non-biological materials within a patient (e.g., on a hardware rather than software level). While B0 shimming using permanent magnets has been demonstrated in a 0.5 T X-ray/MRI hybrid system [46], permanent magnets have not been used to correct susceptibility artifacts caused by non-biological materials within a patient.